1. Field of the Invention
The present invention relates to detection of electromagnetic signals from targeted hidden conductors and, in particular, to the decoupling of interference signals in detected signals from the targeted hidden conductors that result from bleedover of the transmitted signal to adjacent non-targeted conductors.
2. Discussion of Related Art
Underground pipe and cable locators (sometimes termed line locators) have existed for many years and are well known. Line locator systems typically include a mobile receiver and a transmitter. The transmitter is coupled to a target conductor, either by direct electrical connection or through induction, to provide a current signal on the target conductor. The receiver detects and processes signals resulting from the electromagnetic field generated at the target conductor as a result of the current signal, which can be a continuous wave sinusoidal signal provided to the target conductor by the transmitter.
The transmitter is often physically separated from the receiver, with a typical separation distance of several meters or in some cases up to many kilometers. The transmitter couples the current signal, whose frequency can be user chosen from a selectable set of frequencies, to the target conductor. The frequency of the current signal applied to the target conductor can be referred to as the active locate frequency. The target conductor then generates an electromagnetic field at the active locate frequency in response to the current signal.
Different location methodologies and underground environments can call for different active frequencies. The typical range of active locate frequencies can be from several Hertz (for location of the target conductor over separation distances between the transmitter and receiver of many kilometers) to 100 kHz or more. Significant radio frequency interference on the electromagentic field detected by the receiver can be present in the environment over this range. Therefore, receivers of line location systems have often included highly tuned filters to preclude interference from outside sources from affecting the measurement of signals at the desired active locate frequency from the target conductor. These filters can be tuned to receive signals resulting from electromagnetic fields at each of the selectable active locate frequencies and reject signals resulting from electromagnetic fields at frequencies other than the active locate frequencies.
In line location systems, the signal strength parameter determined from detection of the electromagnetic field provides basis for derived quantities of the current signal (i.e., the line current in the targeted conductor), position of the line locator receiver relative to the center of the conductor, depth of the conductor from the line locator receiver, and can also be used as the input to a peak or null indicator (depending on the orientation of the magnetic field to which that the detector is sensitive). All line location systems measure signal strength on one or more measurement channels.
Often in a crowded underground utility environment of metallic pipes and cables, coupling of signals at the active locating frequency from the target conductor to other adjacent underground conductors can occur. These conductors (lines) are not intended to be tracked by the line location system, but coupling of currents from the target conductor to those neighboring conductors through various means (resistive, inductive, or capacitive), termed “bleedover,” can lead a line locator astray such that the operator of the line location system ceases tracking the targeted conductor (e.g., pipe or cable of interest) and instead begins following an adjacent line.
In conventional receivers, it is nearly impossible to determine whether the receiver is tracking the targeted conductor or whether the receiver is erroneously tracking a neighboring conductor. In complicated underground conductor topologies, the effect of interference from electromagnetic fields resulting from bleedover currents in neighboring conductors can result in significant assymetrical electromagnetic fields, which is termed field distortion. Further, conventional systems that attempt to distinguish between the targeted conductor and neighboring conductors typically rely on wireless or wired transmission of phase information from the transmitter, which may be located at such a distance from the receiver of the line locator that receiving such information is impractical.
Therefore, there is a need for line location systems capable of accurately determining the signal strength parameter from the targeted conductor exclusive of neighboring conductors that may provide signals that are a result of inductive or capacitive coupling, using a signal generation and processing method that utilizes only the targeted conductor (pipe or cable) as the transmission medium without relying on a separate communication channel for the transmitter and receiver to share phase information.